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Credibility refers to the trustworthiness of a witness.[1] Credibility evidence comprises evidence admitted for the purposes of strengthening or weakening the testimony of a witness or an accused. Credibility evidence that is used to impeach a witness can be submitted at any point from the time the witness takes the stand. The rule is that a witness always puts their credibility at issue whenever they testify and so it is open to attack. Evidence that bolsters a witness's credibility, however, is not admittable until the credibility of that witness has been impeached.

Whenever testimony is given the trier of fact must determine whether the testimony is to be believed. There is no presumption of honesty among witnesses in a criminal trial[2]

There are several approaches to determining credibility. One school of thought believes that credibility is determined primarily by demeanour and conduct in trial.[3] Another approach says that demeanour and conduct are too subjective,[4]
and so the best approach is to consider the consistency of testimony when compared to reliable facts.[5]

In short, the real test of the truth of the story of a witness in such a case must be its harmony with the preponderance of the probabilities which a practical and informed person would readily recognize as reasonable in that place and in those conditions.

It is generally prohibited to adduce any evidence for the sole purpose of bolstering or increasing a witnesses credibility, suggesting they are more likely to be telling truth. Evidence that is adduced for another reason and also has the effect of bolstering credibility can be permitted. The judge must balance the probative value of the evidence against the prejudicial effect.[1]
Reasons for adducing oath-helping evidence includes showing corroboration with other extrinsic evidence and narrative.

The Crown cannot introduce evidence showing that their witness has testified in court previously resulting in convictions.[2]

The Defence cannot adduce evidence establishing that the accused passed a polygraph test.[3]